The role nitrogen oxides in the formation of acid rain, tropospheric ozone and other environmental hazards has resulted in the imposition of strict standards limiting the discharges of these chemical species. To meet these standards, it is generally necessary to remove at least part of these oxides present in the exhaust gases from stationary or mobile combustion sources.
Denitration or selective catalytic reduction (SCR) technology is commonly applied to combustion-derived flue gases for removal of nitrogen oxides. The denitration reaction comprises the reaction of nitrogen oxide species in the gases, such as nitrogen oxide (NO) or nitrogen dioxide (NO2), with a nitrogen containing reductant, such as ammonia or urea, resulting in the production of diatomic nitrogen (N2) and water.
In addition to nitrogen oxides, sulfur dioxide (SO2) is a chemical species often present in combustion-flue gases that causes environmental concern. Sulfur dioxide present in fossil fuel combustion flue-gases is partly oxidized to sulfur trioxide (SO3) which reacts with water to form sulfuric acid. The formation of sulfuric acid from the oxidation of sulfur dioxide in combustion flue-gases can increase corrosion problems in downstream equipment, can increase power costs associated with air pre-heaters due to the increased temperature required to keep the acid-containing flue-gas above its dew point, and can cause increased opacity in the stack gases emitted to the atmosphere.
Catalyst systems for the removal of nitrogen oxides can increase the amount of sulfur dioxide oxidation since catalytic material utilized in selective catalytic reduction can additionally effectuate the oxidation of sulfur dioxide. As a result, the reduction in the nitrogen oxide content of a combustion flue-gas can have an undesirable side-effect of increasing SO3 formation in the combustion flue-gas.